1. Field of the Invention
The present invention relates to instrumentation gauges of the type used in automotive vehicles, and more specifically to a method for equalizing the magnetic moments produced by the inner and outer coil windings in a rotating magnet measuring instrument.
2. Description of the Prior Art
An electrical gauge of the type utilizing air core coils to obtain pointer movement over a range of 90.degree. and more is favored for inexpensive, highly accurate instrumentation gauges in automotive vehicles.
An outer coil is wound over an inner coil, and the plane of the windings are offset from each other by 90.degree.. The magnetic field produced by one coil operates in conjunction with the magnetic field produced by the other coil. An armature in the form of a permanent magnet is rotatably supported along the center line of the intersection of the magnetic fields from the two coils. As the magnetic fields from the coils change in response to currents through the coils, the permanent magnetic armature is rotated, which in turn causes an angular displacement of a visual pointer attached to the shaft.
Since one coil is wound over the other coil, the outer coil has an effective diameter larger than the inner coil. This difference in diameter causes a difference in the magnetic moment that acts on the permanent magnet armature. It is important to control the magnetic strength emitting from each of the coils, especially with respect to each other, in order to accurately control the position of the gauge pointer. One method of equalizing the magnetic strengths of the two coils would be to wind an additional number of turns in the outer coil to increase the magnetic flux for a given current running through the coil. However, the additional wire added to the coil would increase the resistance of the coil, and therefore would reduce the current flowing through the coil for a given applied voltage.
When integrated circuits are used to drive the inner and outer coils, it is highly desirable to use a standard integrated circuit that employs the same output drivers, gain coefficients, impedances, etc. for the two drive channels. Therefore, it is important for both the inner and outer coils to have the same dc resistance so that the same current will flow through each coil when a given voltage is applied to the terminals of the coils. Because the inner coil is magnetically more efficient in turning the armature and the pointer, it is necessary to provide some other method of equalization if the same length of wire is to be used for both the inner and outer coils.
Allen in U.S. Pat. No. 3,327,208 illustrates a common prior art solution to this problem by employing an adjustable resistor in series with the inner coil in order to reduce the effective current through the more magnetically efficient coil. However, if the driving circuits are to be used for a variety of different coils, it would be necessary to individually adjust and calibrate the variable resistance based upon each coil design, and even for variations in the same coil as manufactured at different locations or on different equipment.
The present invention provides another solution for this problem by equalizing the dc resistance of the two coils through the use of the same conductor length for each coil, while winding a section of the inner coil in reverse direction to reduce the net magnetic efficiency of the coil while maintaining its equivalent dc resistance.